US Secretary of State Hillary Clinton suffered a concussion while battling a nasty stomach bug, officials said Saturday, forcing her to stay home for a second week.

"While suffering from a stomach virus, Secretary Clinton became dehydrated and fainted, sustaining a concussion," Clinton's top aide Philippe Reines said in a statement.

The news came just as Clinton, 65, had been expected to testify on Thursday to US lawmakers about the findings of a highly-anticipated investigation into September's militant attack on the US consulate in Benghazi, Libya.

Clinton has already been out of the public eye for the past week, after she had to cancel a trip to North Africa when she contracted the stomach virus on her return from a five-day visit to Europe.

"The secretary's been really very ill. I mean, very. This stomach virus is a pretty vicious one," State Department spokeswoman Victoria Nuland said Wednesday, adding the condition was not life-threatening.

Wife of ex-president Bill Clinton is the most popular member of President Barack Obama's cabinet, with approval ratings of over 60 percent.

Senator John Kerry, the chairman of the Senate Foreign Relations Committee, is now the odds-on favorite to replace Clinton at the helm of the State Department -- an ambition he has long cherished.

US President Barack Obama has chosen Senator John Kerry to succeed Hillary Clinton as U.S. secretary of state, news networks CNN and ABC reported Saturday based on tips from unnamed sources.

CNN cited a Democratic source who had spoken to Kerry, while ABC mentioned unnamed sources. The White House has not confirmed the reports, according to AFP.